1. Field of the Invention
The present invention relates to the method of neutralizing the residual acidity of phosphoric acid activated carbon derived from cellulosic materials, such as sawdust, ground wood, pulp and bark.
2. Description of the Prior Art
One of several alternative processes for the manufacture of activated carbon from particulate materials comprises the step of combining phosphoric acid with the raw material furnish prior to activation. U.S. Pat. Nos. 1,778,343; 1,875,795; 1,903,834; 1,968,847; 2,008,148; and 3,767,592 are representative disclosures of techniques for activating carbon with phosphorous compounds.
Subsequent to activation, the predominate portion of residual acid is washed from the carbon with fresh water. An economic wash consistent with cyclic recovery of the dissolved acid removes most of the acid except for approximately 5% to 10% titratable H.sub.3 PO.sub.4 as a weight percentage of the carbon. Washing, as the term is used herein, describes that process by which phosphate compounds, most of which are phosphoric acid, on the activated carbon surfaces, are dissolved or otherwise combined with a water flow around the carbon surfaces to be carried away therewith.
For most applications of activated carbon, the foregoing washing step is more than adequate. However, since the water solubility of the residual phosphoric acid is high, aqueous slurries of the carbon product have low pH values. Moreover, when the carbon is used to clarify food products such as corn syrup and sugar liquor, there is evidence to suggest that water soluble phosphate compounds e.g. phosphoric acid, are leached from the carbon into the product. As a result, 0.10 to 10.0 .mu. sized particles are precipitated in the flow stream to cloud the syrup or liquor with a colloidal haze. Although these particulates are insufficient to toxically contaminate the foods treated thereby, the resultant haze defeats the clarification objective of the carbon adsorption step.
It has been determined experimentally that to maintain a 98% clarity in corn syrup and sugar liquor, the maximum quantity of soluble phosphoric acid tolerably present in the carbon is 2% of the carbon weight.
It is, therefore, an object of the present invention to provide a method whereby the residual phosphoric acid found on or interstitially within acid activated carbon pores may be neutralized by conversion to substantially water insoluble compounds. In this context, substantially insoluble shall mean a solubility of less than 2 gms/100 ml of water.
Another object of the present invention is to increase the pH of phosphoric acid activated carbon without resort to soluble buffers such as sodium carbonate.
Another object of the present invention is to bind residual phosphate ion remaining in carbon following activation to and interstitially within the carbon pores in the form of insoluble ash. In this respect, the term bind is used to describe the solid nature of resultant insoluble phosphate compounds and the security of such compounds to remain mechanically or adhesively attached to and within the pore matrix of a carbon particle.
Another object of the present invention is to accomplish the foregoing objects in a procedurally direct and simple manner utilizing commonly available, inexpensive reaction compounds.